1. Field of the Invention
The present invention relates to a zoom lens system, for embodiment, a zoom lens system suited for use as an image reading optical system used in a reading devices such as document copiers and film scanners.
2. Description of the Related Art
When reading a finite distance object, Koeler illumination is often used to uniformly illuminate the object surface. In order to maintain illumination uniformity when Koeler illumination is used, the image forming position of the Koeler illumination system and the entrance pupil position of the finite distance system must be at virtually the same position. Such an optical arrangement is not very complex when the finite distance system is a simple focal lens.
When a zoom lens system is the finite distance system, it is difficult to ensure illumination uniformity when conventional Koeler illumination is used since the entrance pupil position moves during zooming operation. If a well-known front stop zoom lens system is used as a finite distance system, illumination uniformity can be ensured by maintaining a constant entrance pupil position during zooming operation. However, distortion and lateral chromatic aberration correction is difficult because the system has a positive optical power behind the stop in a front stop zoom lens system. Moreover, if these corrections are made during zooming operation, the increase in the zooming ratio becomes complicated, and the zoom lens system is greatly enlarged.